The overall objective or long-term goal of this proposed research is the theoretical and clinical evaluation of the electrical impedence technique as a harmless, non-invasive method of localizing, quantitating, and monitoring intrathoracic fluid volumes (pleural effusion, pericardial effusion, pulmonary engorgement, and pulmonary edema) in man and animals. Accumulations of intrathoracic fluid, which often result from trauma, extensive surgery, shock, and cardiovascular diseases, are presently detected by changes in arterial and central venous pressures, radiography, blood gas analyses, auscultation, and pulmonary compliance. Often these values remain essentially normal until irreversible pulmonary insuffiency or cardiac failure occurs. Preliminary investigations indicate that the impedence technique may reveal intrathoracic fluid accumulations before they are detectable by any of the present methods used either singly or in combination. In addition, certain electrode arrays show promise of providing some degree of localization of fluid accumulation. Accordingly, this investigation has among its immediate goals the following: (1) the determination of the optimal electrode array for the detection of intrathoracic fluid volumes (pleural effusion, pericardial effusion, pulmonary engorgement, and pulmonary edema) in the dog by means of electrical impedance; (2) the determination of the maximum impedance changes produced by each of the above intrthoracic fluid volumes, and to quantitate, separately, each fluid volume in terms of the thoracic impedance change produced while simultaneously assessing the physiological response of the animal by changes in the ECG, heart rate, respiratory rate, and aortic.